Most N-channel Mosfets need their gate voltage to be +10V higher than their source voltage.
BOTH Mosfets used with a IR2113 are N-channel.
The bottom Mosfet is a common source circuit so its source pin is at zero volts and when its gate is at +10V it turns on.
But the top Mosfet is a source follower and for its source to be as high as it can go near the supply voltage then its gate voltage must be +10V higher than the supply voltage.
Bootstrapping is used to doo eet.